1. Field of the Invention
This invention relates to a process for the modification of uniformity in tires, particularly pneumatic radial tires having a carcass ply of organic fiber cords, especially small-size tires such as passenger car radial tires, which effectively and advantageously modifies a radial force variation (hereinafter abbreviated as RFV) and a conicity force (hereinafter abbreviated as COF) of the tire as well as tires applying the above process.
2. Description of Related Art
It is well-known that the uniformity of the tire, particularly the radial tire is an important property depending upon the quality of tire performances. Among uniformity properties, RFV has an influence on performances of almost all radial tires, and COF has particularly a remarkable influence on a straight running performance of the passenger car radial tire. Firstly, RFV will be described below.
When the tire is placed on a uniformity testing machine and run thereon under loading at a state of fixing a loaded radius, the load always varies regardless of a large or small variation during one revolution of the tire, and the quantity of such a variation (all amplitude) is called as RFV. In general, the variation of the load measured by the testing machine can be taken out as a waveform that a primary component forms substantially a sine wave and secondary or more components are superposed thereon when the load applied to the tire is represented on the longitudinal axis and the distance during one revolution of the tire is represented on the horizontal axis. An example of RFV is shown in FIG. 15.
In the tire mounted onto a vehicle, RFV is a variation quantity of reactive force in a radial direction from a road surface per one revolution of the tire running under loading. Therefore, RFV of the tire renders a force for adding vibration to the vehicle, so that the tire having a large value of RFV has problems that the ride comfort of the vehicle against vibrations is degraded and in some cases, an uneven wear is created in the tread rubber to considerably damage the steering stability of the vehicle during the high-speed running.
Therefore, it is necessary to control RFV of the tire within a range not creating the above problems. For this end, all of the tires attaching importance to the uniformity property, particularly the radial tires for passenger cars after the building through vulcanization are subjected to an inspection for sorting acceptance or rejection of uniformity by mounting each of the tires onto a given rim and applying a given internal pressure thereto. As a result, the tires showing a RFV value of more than a given rated value are removed from a forward line as a rejected product. These rejected tires are scrapped or subjected to modification for adjusting the RFV value to a range of the rated value.
And also, the uniformity of the tire includes a longitudinal deflection and a lateral deflection based on the dimensional change in addition to the above force variation. Among these deflections, the longitudinal deflection in the radial direction of the tire particularly affects the tire properties. The absolute value (maximum value) of the deflection in the radial direction is called as radial runout (hereinafter abbreviated as RR) and RR is said to have a close relation to RFV.
After RR is measured together with RFV by the inspection for sorting the acceptance or rejection of uniformity, tires having rejected RFV values are marked at a position showing a maximum value of RR without scrapping as far as possible and removed from the inspection line and subjected to RFV modification. The modification process is a process wherein the RFV-rejected tire is mounted onto a given rim and inflated under a given internal pressure and thereafter the marked surface of the tread rubber is subjected to buffing through a grinder to grind out only a partial area of the tread rubber by a gauge corresponding to RFV value to thereby decrease the RFV value.
Since the waveform of RR does not always correspond to the waveform of RFV as regards peak positions of both waveforms, a process for directly conducting RFV modification through the buffing is carried out in such a partial area along the circumference of the tread portion that centers a position showing a maximum value of RFV in a primary component taken out from the waveform of RFV.
However, even if the RFV value is modified to a proper range by the buffing to relieve the rejected tire according to anyone of the above processes, it is hardly impossible to perfectly repair the appearance of the tread portion subjected to the buffing, so that it is undeniable to lower the worth in the appearance of the tire. Furthermore, it should pay attention to a problem that the powder dust of the rubber caused by the buffing of the tread rubber worsens the working environment.
In JP-T-6-507858 (U.S. Pat. No. 5,616,859) is disclosed a process of modifying the uniformity, wherein RR comes up as a property of the modifying target of the uniformity and a part of a cord in at least one ply at a sidewall portion is subjected to a permanent deformation so as to render RR below an acceptable value and such a permanent deformation restrains a part of the tire other than a position to be modified under an inflation of a previously set pressure to restrict the stretching of the cord in the restrained part.
According to the process disclosed in the above publication, it is certainly possible to conduct the modification of RR within at least an acceptable range without damaging the appearance of the tire. However, the positions showing the maximum value and the minimum value do not always agree between RR and RFV as previously mentioned, and there is frequently caused a case that their positions widely shift each other.
And also, the correlation coefficient between RR and RFV is not so high even in the maximum value and minimum value. Because, RR is a phenomenon based on the unevenness at the cross-sectional shape of the tire, while RFV is a phenomenon based on the change of the rigidity along the circumference of the tire. The unevenness at the cross-sectional shape of the tire is caused by ununiform gauge of the tread portion along the circumference of the tire and the disorder in the arrangement of the constituting members arranged in the tread portion, especially the disorder in the cord arrangement of the cord layer constituting the belt. From this point, it is clear that RFV based on the change of the rigidity along the circumference of the tire is not always coincident with RR.
Moreover, in order to permanently deform a part of the ply cord of the carcass, it is required to apply a very high tension to the ply cord. For this purpose, the internal pressure of the tire should be a very high pressure, so that there is a fear of braking the tire during the permanent deformation of the cord.
Next, COF is a lateral force produced toward the rotating axial direction of the tire by running the tire under loading. A significant point of COF lies in a force generated in a constant direction irrespectively of the rotating direction of the tire. COF is a force mainly generated when the tread portion and the belt arranged therein form a surface of cone frustum shape and corresponds to so-called camber thrust.
COF is measured by using a uniformity testing machine in the same manner as in the measurement of RFV. In the measurement, the lateral force generated from the tire is taken out at a state of dividing into forward rotation and backward rotation of the tire. The lateral force measured is shown in FIG. 16 as a diagram. As shown in FIG. 16, the lateral forces LFP and LFN vary in the rotation of the tire, respectively, and also the direction of generating the lateral force differs between the forward rotation and the backward rotation. In FIG. 16, the lateral force LFP in the forward rotation of the tire is shown at plus side (+) and the lateral force LFN in the backward rotation of the tire is shown at minus side (xe2x88x92).
COF is a direct component of the lateral force and is defined by COF=(LFPW and LFNW)/2 when abstracting the direct component LFPW ( greater than 0) of the lateral force LFP in the forward rotation of the tire and the direct component LFNW ( less than 0) of the lateral force LFN in the backward rotation of the tire with reference to FIG. 16.
If the tires generating a large value of COF are mounted onto right and left sides of an axle so as to offset COF, there is hardly caused an influence upon the straight running property of the vehicle. On the contrary, if plural tires are accidentally mounted so as to generate COF in the same direction, the lateral force of a constant direction is always applied to the vehicle, so that the straight running property of the vehicle is remarkably obstructed.
In general, the modification of COF is carried out by a process wherein an end portion side of the tread rubber in the widthwise direction is subjected to buffing to cut out the tread rubber likewise the modification of RFV. As a process of modifying COF without buffing, the aforementioned JP-T-6-507858 (U.S. Pat. No. 5,616,859) discloses a process wherein permanent strain exceeding an elastic limit is applied to the ply cord of the carcass. However, it is an actual state that both processes for modifying COF have aforementioned problems irrespectively of the buffing or application of permanent strain exceeding the elastic limit to the ply cord.
It is, therefore, a first object of the invention to provide a process for modifying a uniformity of a tire capable of surely and effectively controlling RFV to a range of a given rated value while holding a good appearance without subjecting RFV-rejected tire to cutting work such as buffing or the like and without applying a considerably high internal pressure to the tire and without the danger of tire breakage.
It is a second object of the invention to provide a process for modifying a uniformity of a tire capable of advantageously and largely reducing COF to control COF level to a rated value without applying buffing or a remarkably high internal pressure to COF-rejected tire.
It is a third object of the invention to provide a tire obtained by applying either of the above two processes for the modification of uniformity to RFV-rejected tire and COF-rejected tire to modify proper RFV and COF properties while holding the appearance after vulcanization.
According to a first aspect of the invention, there is the provision of a process for modifying a uniformity of a tire comprising a carcass ply of rubberized organic fiber cords arranged side by side in a radial direction of the tire, which comprises the steps of measuring a radial force variation (RFV) of the tire inflated under a given internal pressure, putting a first distinguishing mark to a first tire portion indicating a minimum value of RFV, putting a second distinguishing mark to a second tire portion indicating a maximum value of RFV, applying different thermal hysteresis to the first tire portion and the second tire portion, and keeping a state of applying the given internal pressure to the tire during the cooling in the thermal hysteresis.
In a preferable embodiment of the first aspect of the invention, a waveform of a primary component is taken out from the measured RFV waveform and the first distinguishing mark is put to the first tire portion corresponding to a maximum value of the primary component waveform, and the second distinguishing mark is put to the second tire portion corresponding to a minimum value thereof. In another preferable embodiment of the first aspect of the invention, it is preferable that the internal pressure of the tire during the heating in the thermal hysteresis is either an atmospheric pressure communicating to air or a weak pressure.
The first aspect of the invention mentioned above includes two ways for modifying RFV. In a first way, at least a sidewall portion of the first tire portion is heated to make a temperature thereof higher than that of the second tire portion at the finish of the heating in the thermal hysteresis. Moreover, the term xe2x80x9cthermal hysteresisxe2x80x9d used herein means a hysteresis ranging from a start of the heating to a finish of the cooling.
In the thermal hysteresis according to the invention, the following two heating means are favorable as regards the heating.
(1) Only at least sidewall portion of the first tire portion is partially heated at a given temperature.
(2) At least sidewall portions of the first tire portion and the second tire portion are heated at different amounts of heat, respectively.
It is favorable that the heating is carried out so as to make an internal temperature of at least sidewall portion of the first tire portion higher by no less than 40xc2x0 C. than that of at least sidewall portion of the second tire portion at the finish of the heating of the tire.
Further, as the cooling in the thermal hysteresis according to the invention, the following two cooling means are favorable.
(3) The whole of the tire subjected to the heat is spontaneously cooled.
(4) At least sidewall portion of the first tire portion is slowly cooled at a cooling rate slower than that of at least sidewall portion of the second tire portion.
The term xe2x80x9cspontaneous coolingxe2x80x9d used herein means that the surroundings of the tire are cooled while being exposed at room temperature or an atmospheric temperature.
In a second way of the RFV modification process, the whole of the tire is uniformly heated at the same temperature and at least sidewall portion of the second tire portion in the tire after the finish of the heating is cooled at a cooling rate faster than that of at least sidewall portion of the first tire portion.
As to the cooling, it is favorable that at least sidewall portion of the second tire portion is quenched by force-cooling and at least sidewall portion of the first tire portion is slowly cooled by spontaneous cooling.
According to a second aspect of the invention, there is the provision of a process for modifying a uniformity of a tire having a carcass ply of rubberized organic fiber cords arranged side by side in the radial direction, which comprises the steps of measuring a quantity and a direction of a conicity force (COF) of the tire inflated under a given internal pressure, putting a third distinguishing mark to a first half tire part located in COF generating direction when the tire is divided into first and second half tire parts with respect to an equatorial plane of the tire, and applying different thermal hysteresis to the first half tire part and the second half tire part, and applying the given internal pressure to the tire during a period ranging from a middle stage to a last stage of a cooling in the thermal hysteresis.
In a preferable embodiment of the second aspect of the invention, the thermal hysteresis is carried out by uniformly heating the whole of the tire, and cooling the first half tire part having the third distinguishing mark at a temperature lower than that of the second tire half-part in at least an initial stage of the cooling of the tire.
In another preferable embodiment of the second aspect of the invention, the internal pressure of the tire is made either an atmospheric pressure or a weak pressure during a period ranging from an initial stage to a middle stage of the cooling in the thermal hysteresis.
In the other preferable embodiment of the second aspect of the invention, the first half tire part is cooled at a cooling rate faster than that of the second half tire part during a period ranging from an initial stage of the cooling of the tire to at least an application of the given internal pressure to the tire.
In order to apply different cooling rates, it is preferable that the first half tire is quenched by force-cooling and the second half tire part is slowly cooled by spontaneous cooling.
As to the heating in the thermal hysteresis, it is favorable that the internal pressure of the tire during the heating is made either an atmospheric pressure communicating to air or a weak pressure.
According to a third aspect of the invention, there is the provision of a pneumatic tire applied to the process for modifying the tire uniformity relating to RFV mentioned above, in which in a carcass ply of organic fiber cords arranged side by side in the radial direction, a modulus of elasticity of the organic fiber cord located in at least sidewall portion of the first tire portion is larger than that in at least sidewall portion of the second tire portion.
According to a fourth aspect of the invention, there is the provision of a pneumatic tire applied to the process for modifying the tire uniformity relating to COF mentioned above, in which in a carcass ply of organic fiber cords arranged side by side in the radial direction, a modulus of elasticity of the organic fiber cord located in the first half tire part is smaller than that in second half tire part.
In the tires according to the third and fourth aspects of the invention, the difference in the elastic modulus of the organic fiber cord is a significant of at least 5% in a significant examination using a statistical method.
The invention will be described with reference to the accompanying drawings, wherein:
FIG. 1 is a diagrammatically left-half section view of a first embodiment of the pneumatic tire according to the invention;
FIG. 2 is a schematically plan view of a main part of an apparatus for heating a tire;
FIG. 3 is a diagrammatically section view taken along a line IIIxe2x80x94III of FIG. 2;
FIG. 4 is a schematically plan view of a main part illustrating a modified embodiment of the heating apparatus shown in FIG. 2;
FIG. 5 is a diagrammatically section view taken along a line Vxe2x80x94V of FIG. 4;
FIG. 6 is a schematically plan view of a simplified apparatus for cooling a tire;
FIG. 7 is a diagrammatically section view taken along a line VIIxe2x80x94VII of FIG. 6;
FIG. 8 is a schematically plan view of another apparatus for cooling a tire;
FIG. 9 is a diagrammatically section view taken along a line IXxe2x80x94IX of FIG. 8;
FIG. 10 is a diagrammatically section view-of an apparatus for heating and cooling a tire;
FIG. 11 is a diagram of a primary component of RFV;
FIG. 12 is a diagram before and after RFV modification by a first way in RFV modifying process according to the invention;
FIG. 13 is a diagram before and after RFV modification by a second way in RFV modifying process according to the invention;
FIG. 14 is a diagram of lateral force in forward and backward rotations of a tire by applying a process for modifying COF according to the invention;
FIG. 15 is a diagram of RFV before modifying; and
FIG. 16 is a diagram of lateral force in forward and backward rotations of a tire before modifying.